There are several techniques employed for surface-mounting integrated circuits (ICs) onto substrates such as printed circuit boards (PCBs). One common technique is the use of Ball Grid Arrays (BGAs) in which the pins of the IC are replaced by solder balls disposed on the underside of the IC package with the PCB carrying conductive pads in a pattern corresponding to the solder balls. In a technically-equivalent variation of this technique, the solder balls are formed instead on the PCB itself in a pattern that matches the pins of the IC.
Mounting the IC package to the PCB involves placing the IC in the correct position on the PCB substrate and then heating the assembly, for example by means of an oven or infrared heater, causing the solder balls to melt in a process called “reflow”. During reflow, the IC is drawn down or sinks towards the substrate in two stages: a first drop caused by the surface tension of the melting solder and a second drop due to the subsequent contraction of the solder mass as it solidifies during cooling. Surface tension within the molten solder also tends to retain the package in the correct alignment on the substrate. The solder then cools and solidifies to form the desired electrical contacts.
IC packages using a similar solder-ball technology are known as “flip-chips”, wherein the solder balls are deposited onto the top-side of the chip or wafer which is then flipped over and aligned with the matching pads on the PCB. The connections are again created by heating and melting the solder during the reflow process.
BGA packages have a number of advantages over conventional pin grid array (PGA) techniques, such as improved packaging and assembly and improved conductive and thermal performance. As a result, BGAs are becoming widely used globally, particularly for larger PCBs with many connections since they allow for higher density components.
On the other hand, as with many electronic systems, the soldering process is not wholly reliable and open contacts or short circuits maybe formed inadvertently. In particular, since the solder balls are substantially non-compliant (that is, unable to flex), thermal or mechanical stresses caused, for example, by differences in coefficients of thermal expansion between the PCB substrate and the BGA package or by flexing and vibration of the PCB, may cause the solder joints to fracture.
It is therefore conventional to perform some form of testing of the IC connections on the PCB after soldering. In order to maintain production efficiency, an automated testing process is generally preferred. The automated inspection of electronic components is an integral part of electronic assembly manufacture to prevent and detect faults and to ensure quality and performance of the assembled systems. Increases in PCB complexity and the desire to improve yields have required the development of real-time automated inspection.
For example, visual inspection of a PCB can be performed using Automated Optical Inspection (AOI) in which a video camera is used to scan the assembly and compare the scanned image with pre-recorded images of properly soldered PCBs. Where conventional PGA soldering methods are used to mount the IC to the PCB, AOI can be used to determine the efficacy of the soldered joints. In the case of BGA-mounted ICs, however, AOI systems are usually unable to measure the integrity of the solder joints due to the position of the connections on the underside of the IC package and are therefore generally limited to diagnosing missing components and placement errors. This may lead to faults not being diagnosed using standard automated inspection techniques, resulting in faulty products and a loss of revenue for manufacturers.
Co-pending PCT Application No. PCT/EP2010/053747 in the name of the present applicants, the contents of which are expressly incorporated herein by reference, addresses this problem and discloses an automated inspection process in which a downwardly-directed laser measurement device positioned directly overhead the IC package detects the height of the package above the substrate before and after reflow. Specifically, the laser measurement device is positioned a predetermined height above the substrate and emits a beam substantially vertically downwards which is reflected from the top surface of the package onto an associated CCD camera, allowing the distance of the package from the device to be calculated. A second beam is then emitted and reflected from the surface of the substrate adjacent the package allowing the distance of the substrate from the device to be calculated.
Subtracting the first distance from the second gives the height of the top surface of the package from the substrate.
By calculating the height of the upper surface of the package above the substrate both before and after reflow, the drop of the package due to the reflow process can be calculated. The applicants have identified that a correctly soldered component will drop a predetermined amount during reflow. If one or more solder balls has not completely melted during soldering, then the component will remain supported without settling to the expected standoff height. If the measured drop is outside the predetermined range, the component is deemed to be mounted incorrectly. This method has been found to result in a high degree of accuracy in detecting improperly soldered packages.
It is against this background that the present invention has been conceived. While the applicants' prior technique produces more accurate results than previous methods, it is nevertheless an aim of the present invention to provide an improved technique for verification of the solder reflow process for soldered components, particularly BGA-soldered ICs, and to further increase accuracy. Embodiments of the invention may provide a method and/or apparatus that employs a conventional laser measuring device in a highly unconventional way to measure the height of a BGA-mounted IC or other package on a substrate.
The applicants have found that the claimed method and apparatus produces exceptionally and unexpectedly high accuracy, greatly surpassing the theoretical capabilities of the equipment employed. This enables the integrity of the soldered joints to be determined more quickly, more accurately and more cost-effectively in an automated process. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.